1. Field of the Invention
The present invention relates to a display apparatus, such as a liquid crystal display apparatus or the like, which comprises a plurality of display pixel electrodes two-dimensionally arranged, and a method for producing the same. The present invention also relates to an active substrate for use in the display apparatus.
2. Description of the Related Art
Conventionally, the above-described display apparatuses include, for example, liquid crystal display apparatuses as well as EL display apparatuses, plasma display apparatuses, and the like. For example, a plurality of pixel portions arranged in a matrix can be selectively driven to display a desired display pattern (image) on a display screen with high density.
As a technique of selecting the pixel portions, an active drive technique is known, in which: individually separated pixel electrodes are arranged in a matrix of columns and rows; a switching element is connected to each pixel electrode; and the pixel electrodes are selectively driven. Examples of a commonly used switching element for selectively selecting a plurality of pixel electrodes, include a TFT (thin film transistor) element, an MIM (metal-insulator-metal) element, a MOS transistor element, a diode, and the like. By using such a switching element to selectively drive the pixel electrodes, various display media, such as a liquid crystal, an EL light emitting layer, a plasma light emitting material, or the like, which is interposed between the pixel electrode and a counter electrode facing thereto, are driven optically, so that a display pattern may be viewed. Such an active drive technique is capable of high contrast display, and has been practically utilized in a liquid crystal televisions, computer terminal display apparatuses, and the like.
FIG. 19A is a top view showing a configuration of a basic unit on one of a pair of substrates, which is an active matrix substrate, in a conventional active type liquid crystal display apparatus. The substrates have a liquid crystal layer interposed between them. FIG. 19B is a cross-sectional view, taken along line X-X′ in FIG. 19A, FIG. 19C is a cross-sectional view, taken along line Y-Y′ of FIG. 19A.
In the conventional active type liquid crystal display apparatus of FIGS. 19A to 19C, the pair of substrates are an active matrix substrate 100 and a counter substrate, which face each other and have a liquid crystal layer (display medium) interposed between them. In the active matrix substrate 100, a plurality of gate bus lines 1 (scanning lines) are parallelly arranged in predetermined intervals on a glass substrate 10, extending horizontally in FIG. 19A, while a plurality of source bus lines 2 (signal lines) are parallelly arranged at predetermined intervals, extending vertically in FIG. 19A and intersecting the gate bus lines 1 (e.g., at right angles). Thus, the gate bus lines 1 and the source bus lines 2 are arranged in a grid (matrix). A pixel electrode 3 (a portion enclosed with a dashed line in FIG. 19A) made of a transparent electrode is provided in each region surrounded by adjacent gate bus lines 1 and adjacent source bus lines 2 (or at an intersection between the gate bus line 1 and the source bus line 2).
As shown in FIG. 19A, a dual-gate TFT 4 serving as a switching element is provided in a portion projecting from the gate bus line 1. The TFT 4 comprises a semiconductor layer made of silicon (Si), which is provided on the glass substrate 10 via a base coat film 11, as shown in the cross-sectional view of FIG. 19B taken along line X-X′ of FIG. 19A. In this semiconductor layer, a channel region 12a, source/drain regions (e.g., an n+ Si layer) 12c, and an LDD region (e.g., an n− Si layer) 12b are provided. The source/drain regions 12c are formed by adding a high-concentration impurity to opposite sides of the channel region 12a. The LDD region 12b is formed by adding a low-concentration impurity between the channel region 12a and the source/drain regions 12c. A gate electrode 1a projecting (branching) from the gate bus line 1 is provided on the channel region 12a via a gate insulating film 13. Over the entire above-described structure, a pixel electrode 3 is provided via an interlayer film 14 and a resin layer 15. An alignment film (not shown) made of polyimide (PI) is provided on the pixel electrode 3. A liquid crystal layer is provided on and comes into contact with the alignment film (PI).
As shown in FIG. 19A, an additive-capacitor bus line (additive-capacitor line) 5 made of a metal layer (gate metal), which is patterned in the same step as that in which the gate bus line 1 is patterned, is disposed for each gate bus line 1 and in parallel to the gate bus line 1. A multilayer structure of the additive-capacitor portion is shown in the cross-sectional view of FIG. 19C taken along line Y-Y′ of FIG. 19A. In the multilayer structure, a semiconductor layer (extending portion 12) extending from the drain region 12a of the TFT 4 is provided via the gate insulating film 13 below a broad-width portion 5A of the additive-capacitor bus line 5. The extending portion 12 of the semiconductor layer is connected to the pixel electrode 3 via a metal layer (source metal) 6, which is patterned in the same step as that in which the source bus line 2 is patterned, the connection is at a contact hole portion 6A which penetrates through the gate insulating film 13, the interlayer film 14 and the resin layer 15. As a result, the extending portion 12 (one additive-capacitor electrode) and the broad-width portion 5A (the other additive-capacitor electrode) face each other, having the gate insulating film 13 interposed between them. An additive capacitor is established between the extending portion 12 and the broad-width portion 5A.
In the thus-constructed conventional active type liquid crystal display apparatus, for example, when the TFT 4. (switching element) is a defective element, a signal voltage that should be otherwise input is not supplied to the pixel electrode 3 connected to the defective element. As a result, the user recognizes the defective element as a dot-like pixel defect (hereinafter referred to as a point defect) on a display screen. Such a point defect significantly impairs the display quality of a liquid crystal display apparatus, raising a problem with the production yield.
There are roughly two major reasons for the pixel defect.
One reason is that the defective TFT 4 prevents the pixel electrode 3 from being sufficiently charged by an image signal from the source bus line 2 within a time when the TFT 4 is selected by a scanning signal (signal from the gate bus line 1). Such a defect is hereinafter referred to as an ON defect). The other reason is that when the defective TFT 4 is not selected, charges on the pixel electrode 3 leak due to the defective TFT 4. Such a defeat is hereinafter referred to as an OFF defect.
The ON defect is caused by a defect of the TFT 4 (switching element), while the OFF defect can have two causes: electrical leakage through the TFT 4 (switching element); and electrical leakage between the pixel electrode 3 and the bus lines 1 and 2. In either case of the ON defect or the OFF defect, a voltage applied between the pixel electrode 3 and the counter electrode (not shown) no longer reaches a required display voltage value. Therefore, a pixel defective portion is viewed as a luminous point in the normally white mode (a display mode in which the light transmittance is maximized when a voltage applied to a liquid crystal layer is zero), while a pixel defective portion is viewed as a black point in the normally black mode (a display mode in which the light transmittance is minimized when a voltage applied to a liquid crystal layer is zero).
Such a point defect can be visually detected by an inspector as follows. When a counter substrate having a counter electrode is attached to the active matrix substrate 100 having the TFT 4 (switching element) and a gap therebetween is filled with liquid crystal, a predetermined electric signal (detection signal) is applied to both of the bus lines 1 and 2, so that a point defect may be seen by the inspector. Such a point defect can be repaired by, for example, short-circuiting the source bus line 2 and the pixel electrode 3 no matter whether or not the gate bus line 1 is selected. In this case, a signal voltage supplied from the source bus line 2 is used to charge and discharge the pixel electrode 3.
However, in the conventional example of FIGS. 19A to 19C, it is difficult to perform the above-described repair due to the arrangement of the source bus line 2 and the pixel electrode 3. As a result, a product having many point defects must be discarded, leading to poor production yield and high production cost.
A liquid crystal display apparatus in which repair of such a point defect is possible has been proposed in Japanese Laid-Open Publication No. 4-265943. The liquid crystal display apparatus of Japanese Laid-Open Publication No. 4-265943 will be described with reference to FIGS. 20A and 20B.
FIG. 20A is a top view showing an exemplary structure of a basic unit in an active matrix substrate, which is one of a pair of substrates facing each other and having a liquid crystal layer interposed between them, in a conventional active type liquid crystal display apparatus. FIG. 20B is an enlarged view showing the portion enclosed by a circle in FIG. 20A. Note that parts of FIG. 20A having substantially the same functions and effects as those of FIG. 19A are indicated by the same reference numerals and will not be explained in detail.
As shown in FIG. 20A, the active type liquid crystal display apparatus comprises: a gate bus line projecting portion 21 projecting from a gate bus line 1 toward the inside of a pixel electrode 3; and a source bus line projecting portion 22 projecting from a source bus line 2 toward the inside of the pixel electrode 3. The projecting portions 21 and 22 overlap each other via an insulating film. A conductor piece 23 is provided at a tip portion of the gate bus line projecting portion 21 via the insulating film. While the conductor piece 23 is electrically connected to the pixel electrode 3, the gate bus line projecting portion 21 is not electrically connected with the pixel electrode 3 due to the insulating film. Also, the source bus line projecting portion 22 is not electrically connected with the gate bus line projecting portion 21 due to the insulating film.
For a pixel portion in which a point defect has been detected, a root portion of the gate bus line projecting portion 21 is irradiated with laser to electrically separate (cut) the gate bus line projecting portion 21 from the gate bus line 1 to achieve insulation, as shown in a portion A enclosed with a dashed line in FIG. 20B. Next, in a portion B enclosed with a dashed line, the insulating film between the source bus line projecting portion 22 and the gate bus line projecting portion 21 is destroyed by laser irradiation to short-circuit the source bus line projecting portion 22 and the gate bus line projecting portion 21. Further, in a portion C enclosed with a dashed line, the insulating film between the gate bus line projecting portion 21 and the conductor piece 23 connected to the pixel electrode 3 is destroyed by laser irradiation to short-circuit the gate bus line projecting portion 21 and the conductor piece 23. By performing laser irradiation three times, electric conduction is established between the source bus line 2 and the pixel electrode 3, so that the defective pixel has an average luminosity of all pixels, i.e., the point defeat is repaired.
Japanese Laid-Open Publication No. 4-278927 discloses a liquid crystal display apparatus in which is possible of a defective pixel caused by a defect, such as a pinhole or the like, which occurs in an additive-capacitor electrode 5B (FIG. 21). The active type liquid crystal display apparatus of Japanese Laid-Open Publication No. 4-278927 will be described below with reference to FIG. 21.
FIG. 21 is a top view showing an exemplary structure of a basic unit in an active matrix substrate, which is one of a pair of substrates facing each other and having a liquid crystal layer interposed between them, in another conventional active type liquid crystal display apparatus. Note that parts of FIG. 21 having substantially the same functions and effects as those of FIG. 19A are indicated by the same reference numerals and will not be explained in detail.
In the active type liquid crystal display apparatus of FIG. 21, an additive-capacitor bus line 5 is provided adjacent to a pixel electrode 3 and parallel to a gate bus line 1. A portion facing the TFT 4 formation portion of the additive-capacitor bus line 5 overlaps an end portion of a first conductor 25 via an insulating film. The other end portion of the first conductor 25 overlaps an end portion of a second conductor 26 via a gate insulating film, the end portion of a second conductor 26 being disposed under the other end portion of the first conductor 25. An end portion of a projecting portion 27 of the source bus line 2 overlaps the other end portion of the second conductor 26 via the gate insulating film, the end portion of a projecting portion 27 of the source bus line 2 being disposed over the other end portion of the second conductor 26. The additive-capacitor electrode 5B (hatched portion) facing the additive-capacitor bus line 5 is connected to the pixel electrode 3 over the first conductor 25.
By irradiating a basic portion of the first conductor 25, the overlapping portion of the first conductor 25 and the second conductor 26, and the overlapping portion of the second conductor 26 and the projecting portion 27 of the source bus line 2 with laser light through a glass substrate, the source bus line 2 and the pixel electrode 3 are short-circuited and the additive-capacitor electrode 5B is cut off from the pixel electrode 3, thereby repairing a defective pixel portion.
However, the above-described active type liquid crystal display apparatus of Japanese Laid-Open Publication No. 4-265943 requires performing laser irradiation three times: (1) laser irradiation (the portion A indicated with a dashed line) for electrically separating the gate bus line projecting portion 21 from the gate bus line 1 in order to short-circuit the source bus line 2 and the pixel electrode 3 to repair a defective pixel portion caused by a defective TFT; (2) laser irradiation (the portion B indicated with a dashed line) for short-circuiting the gate bus line projecting portion 21 and the source bus line projecting portion 22; and (3) laser irradiation (the portion C indicated with a dashed line) for short-circuiting the gate bus line projecting portion 21 and the pixel electrode 3. Therefore, it is difficult to repair a defective pixel portion caused by a defective TFT. In addition, the defective TFT cannot be cut off from the pixel electrode 3, and therefore, it is not possible to perform repair depending on the type of pixel defect.
In the above-described active type liquid crystal display apparatus of Japanese Laid-Open Publication No. 4-278927, a defective pixel portion caused by the defect of the additive-capacitor electrode 5B is repaired by performing laser irradiation two times to short-circuit the source bus line 2 and the pixel electrode 3 and performing laser irradiation once to cut off the pixel electrode 3 from the additive-capacitor electrode 5B. Thus, although the pixel electrode 3 is cut off from the additive-capacitor electrode 5B, it is difficult to repair a defective pixel portion caused by a defective additive-capacitor portion.